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Clinton Gormley


Elastic::Manual::NoSQL - Differences between relational DBs and NoSQL document stores


version 0.52


Elasticsearch (and any NoSQL document store) is quite different from a relational database, so you need to think about information storage and retrieval in a different way. Some differences are:

It is not relational

Elasticsearch stores individual documents/objects. All queries look at each document in isolation to decide if that document matches the query, and is either included or excluded. There is no way of doing real JOINs like:

    SELECT * FROM post, user
    WHERE user.name = 'John'
      AND user.id   = post.user_id

Instead you could collect all the IDs of users whose name is "John", then search for posts who have a matching user_id. This approach works for some things, but in this case, probably wouldn't scale.

(There is a pseudo relationship available in Elasticsearch, called "parent-child", which allows you to return parent documents based on the content of child documents. This can be useful in certain circumstances.)

Denormalize your data

In relational databases, you try to normalize your data, so that data is not repeated. But because NoSQL is not relational, you HAVE to repeat your data. For instance, to run a query like the above (ie Give me posts by an author whose name is "John", you could store the author's name with every post object. With this data structure, you can just look at post objects to find the ones you are after. No joins necessary.

This also means that if the user changes their name, you would need to update all of their posts with the embedded information.

No transactions or locking

Elasticsearch is a distributed system, which makes it very scalable. But introducing pessimistic concurrency control (locking) into a distributed system adds a single point of failure and concurrency issues.

Instead, it uses optimistic concurrency control: every document has a current version number, which gets updated on every change. If you try to make a change to an older version of a doc, you get a conflict error, and you can use the strategy appropriate to your context to resolve the conflict.

There are no transactions either. While creating, updating or deleting a single document is atomic, you can't change multiple documents in one transaction which can be rolled back if any of the changes fails. Your application logic needs to take this into account.

While CRUD (create, retrieve, update, delete) actions are real time (eg as soon as you have created a document, you can retrieve it again), search is NEAR real time. By default, document changes become visible to search within one second.

For example, consider a website allowing a user to create blogs posts:

  1. The user submits a new post

  2. We create the post in Elasticsearch, and return a list of all the user's posts, sorted by date.

  3. The new post is missing!

Again, you need to design your application accordingly. You could:

  • Retrieve the list of the user's posts asynchronously from the browser, and build in a one second delay, to ensure that the new post will be included.

  • Don't refresh the list of posts. Instead, just add the new post to the existing list using Javascript

  • When querying for all posts, deliberately exclude the new post ID, then add it manually to the front of the list


These differences may sound a bit daunting, but really they are quite easy to manage. It takes a little time to understand the practicalities, but once you do, it will all seem quite natural.

The benefits that Elasticsearch brings (eg amazing full text search and easy scaling) makes this all worthwhile.



Clinton Gormley <drtech@cpan.org>


This software is copyright (c) 2015 by Clinton Gormley.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.